Fuel efficiency of internal combustion engines can be substantially improved by varying the displacement of the engine in response to the demanded torque. Full displacement allows for the full torque to be available when required, yet using a smaller displacement when full torque is not required can significantly reduce pumping losses and improve thermal efficiency. The most common method today of implementing a variable displacement engine is to deactivate a group of cylinders substantially simultaneously. In this approach the intake and exhaust valves associated with the deactivated cylinders are kept closed and no fuel is injected when it is desired to skip a combustion event. For example, an 8-cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three displacements.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next. Skip fire engine operation is distinguished from conventional variable displacement engine control in which a designated set of cylinders are deactivated substantially simultaneously and remain deactivated as long as the engine remains in the same variable displacement mode. Thus, the sequence of specific cylinders' firings will always be the same for each engine cycle during operation in a variable displacement mode (so long as the engine remains in the same displacement mode), whereas that is often not the case during skip fire operation. For example, an 8-cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders.
In general, skip fire engine operation facilitates finer control of the effective engine displacement than is possible using a conventional variable displacement approach. For example, firing every third cylinder in a 4-cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice most implementations restrict operation to a set of available firing fractions, sequences or patterns. The Applicant has filed a number of patents describing various approaches to skip fire control. By way of example, U.S. Pat. Nos. 8,099,224; 8,464,690; 8,651,091; 8,839,766; 8,869,773; 9,020,735; 9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,200,587; 9,291,106; 9,399,964, and others describe a variety of engine controllers that make it practical to operate a wide variety of internal combustion engines in a dynamic skip fire operational mode. Each of these patents is incorporated herein by reference. Many of these patents relate to dynamic skip fire control in which firing decisions regarding whether to skip or fire a particular cylinder during a particular working cycle are made in real time—often just briefly before the working cycle begins and often on an individual cylinder firing opportunity by firing opportunity basis.
In some applications referred to as dynamic multi-level skip fire, individual working cycles that are fired may be purposely operated at different cylinder outputs levels—that is, using purposefully different air charge and corresponding fueling levels. By way of example, U.S. Pat. No. 9,399,964 describes some such approaches. The individual cylinder control concepts used in dynamic skip fire can also be applied to dynamic multi-charge level engine operation in which all cylinders are fired, but individual working cycles are purposely operated at different cylinder output levels. Dynamic skip fire and dynamic multi-charge level engine operation may collectively be considered different types of dynamic firing level modulation engine operation in which the output of each working cycle (e.g., skip/fire, high/low, skip/high/low, etc.) is dynamically determined during operation of the engine, typically on an individual cylinder working cycle by working cycle (firing opportunity by firing opportunity) basis. It should be appreciated that dynamic firing level engine operation is different than conventional variable displacement in which when the engine enters a reduced displacement operational state, a defined set of cylinders are operated in generally the same manner until the engine transitions to a different operational state.
The fuel control system in many engine controllers includes an air charge estimator, such as a MAC (mass air charge) estimator or an APC (air per cylinder) estimator. Conventional air estimation techniques are often not particularly well suited for use in skip fire controlled engines due to the impacts of irregular and/or shifting firing sequences that can occur during skip fire operation. Many prior art air charge estimators utilize a mean or a filtered value of a measured absolute manifold pressure (MAP) in their air charge estimations. Often, use of mean pressures will not accurately predict the cylinder air charge on a firing-by-firing basis because previous MAP values are not necessarily indicative of the future trajectory of the manifold pressure in a skip firing engine. In addition, changes in the position of other airflow control actuators such as throttle and cam phasers taking place after the cylinder air estimate is required for fuel injection amount determination are not accounted for. Fuel injection based on poorly predicted air charges will result in a mixture of rich and lean combustions, causing poor catalytic converter efficiency, and when extreme, loss in torque and efficiency as combustion of the excess fuel in rich-firing cylinders will be completed inside the catalytic converter.
U.S. patent application Ser. No. 13/794,157, U.S. Provisional Patent Application No. 62/068,391 and SAE Technical Paper 2015-01-1717 each describe air charge estimation techniques suitable for use in skip fire engine operation. Each of these references is incorporated herein by reference. Although such estimators work well in many applications, there are continuing efforts to further improve the air charge estimators and estimation techniques in manners that are well suited for use in skip fire and multi-level engine control schemes.